Conventional power plugs are substantially composed of a insulating material, except for the conducting pins which are exposed so that they may be inserted into a socket when in use. There is no problem for such plugs to be employed with 110 voltage. As to 220 voltage or more, however, even with the slightest initial contact of the pins with the socket, a high contact voltage or electrical sparks may be generated, which in fact will shock a person who has a sight of the sparks while his hand at that time is pushing the plug, or furthermore, will kill the person if his hand is close enough to the pins.